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ISDN – Integrated Services Digital Network

Today my topic is ISDN – Integrated Services Digital Network. ISDN is a digital version of traditional analog PSTN. Why do we need digital? What is the typical interface at home as well as in office?
There are two versions of ISDN? What are their differences in setup and transmission rate?
It becomes a legacy technology, still, ISDN has its place, as backup to dedicated lines, and in locations where broadband service is not yet available.
Keywords: ISDN, PRI, BRI, primary rate Interface, basic rate interface, analog, digital, PSTN.
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Today my topic is ISDN – Integrated Services Digital Network
Not long time ago, the PSTN was all analog, from customer premises to central office (CO) – local loop – and from central office to the backbone of the network. PSTN was simply designed for analog telephone calls.
With the advent of personal computers, digital data transmission was needed. For voice calls, everything remained the same. For computers, modems were developed to allow digital exchanges over existing analog PSTN network. Digital signals are modulated to analog signals and transmitted over the PSTN network. Only at the receiving end, analog signals are demodulated back to original digital signals.
But analog signals have many disadvantages compared with digital signals. 1) over the long-distance, analog signals cannot maintain high quality; 2) analog signals carry less information per second than digital signals; 3) analog signals are not so flexible as digital signals in terms of data rate services and support. For these reasons, ISDN came to the rescue.
ISDN is a set of international communication standards designed in the 1980s and improved in the 1990s. It is a digital network to transmit voice, image, video, and text over the existing circuit-switched PSTN telephone network.
ISDN provides a single interface for hooking up your telephone, fax machine, and computers.
Here is a simplified version of the ISDN set up at a small office/home office (SOHO). Different types of devices, such as an analog phone, a fax machine, or a computer, are connected to an ISDN terminal adapter, or TA, which converts different types of data into ISDN format so that they can share the same digital PSTN network.
There are two ISDN options: Basic Rate Interface (BRI), and Primary Rate Interface (PRI).
BRI (Basic Rate Interface)
BRI uses two bearer channels and one 16 Kbps data channel, represented by the notation 2B+D. Bearer channel, by its name, bears traffic from point to point. It uses a circuit-switching method to carry data. Each bearer channel can carry data at the rate of 64 Kbps.
Data channel might be confusing because the data channel does not carry the real data, instead, it carries information about data and signaling information. Take a phone call as an example, data channel may carry information like session initiation and termination signals, caller ID, and etc.
BRI is a common option for home users.
This is an example of BRI set up at a small office/home office. Three different types of data are converted to ISDN formats and then connected to ISDN through Network Termination 1 (NT1).
NT1 controls electrical and physical termination of ISDN at the user’s premise. Terminal adapter and NT1 are not necessarily two separate devices. They can be in one single box.

PRI (Primary Rate Interface)
PRI is commonly used by businesses and organizations. This type of ISDN uses 23 bearer channels and one 64-Kbps Data channel, represented by the notation 23B+D.
In a typical PRI setting, there is an extra device: Network Termination 2, or NT2, which is a multiplexer of different data. NT1 is then connected to the ISDN network.
In the last 10 years, ISDN has been replaced by broadband internet access connections like DSL, Cable Modems, wireless, and other less expensive but faster technologies. Still, ISDN has its place, as backup to dedicated lines, and in locations where broadband service is not yet available.